Iknow I mentioned in my last severe weather article that I would be doing a detailed discussion about tornadoes sometime soon, but that sometime is not this week. Instead, I’ve decided to look at a particular aspect of thunderstorms that everyone has seen, can be deadly but rarely is, and is probably one of the best things about thunderstorms: I’m talking about lightning.
Lightning, at least in our part of the world, can only occur in a thunderstorm/ shower, and is probably the most recognized aspect of thunderstorms. In this article we’re going to begin our look at lightning by taking a detailed look at how lightning forms, but before we take our technical look at lightning let’s dispel the fist incorrect notion about lightning: that you can get lightning without a thunderstorm.
Growing up I have seen and been told about a particular type of lightning that appears to occur without any thunderstorms around, called “heat lightning.” You can see this type of lightning on a hot summer evening, usually during a dry period. You will see a flash of lightning off in the distance but you can’t make out any thunderstorm clouds and usually you can’t hear any thunder. What’s happening is that a thunderstorm is actually producing this lightning, but it’s far enough away that you can see the flashes of lightning but you can’t hear
the thunder. Due to the hot weather, the air or sky is often hazy, which prevents the observer from clearly making out the thunderstorm. Instead, they simply see clear skies overhead slowly blending into the haze and then the clouds of the thunderstorm. Finally, because this type of lightning usually occurs during a dry spell, these thunderstorms will often be dry thunderstorms or those that do not produce any rain. So, the next day, when people are talking about the “heat lightning,” they connect with the idea that there was no rain anywhere around so therefore, there must not have been any thunderstorms.
OK, now on to a bit of a technical look at lightning. For this discussion we’ll look at a typical lightning strike that starts in the cloud and hits the ground. First of all, lightning is caused by a build
of electrical charge within a thunderstorm. Strong up-and-down drafts within a thunderstorm cause particles of dust, water and ice to hit each other. These millions of collisions allow for electrons to be transferred between particles, causing these particles to become charged. This is a very similar process to the one that gives you a charge when you drag your feet across a carpet in the winter. Within the thunderstorm these same up-and-down drafts separate these charged particles into regions, so some areas of the storm become negatively charged and other areas positively charged. Exactly how this happens is still not completely understood.
When an area of the storm gains a strong enough charge it will act on the air around it causing it to ionize (the air molecules break apart, forming positive and negative atoms). This ionized air can now conduct electricity. Meanwhile on the ground, the strong negative charge in the clouds above the earth pushes electrons (which are negative) away. In objects that are good conductors (metals) the electrons move easily, so these objects become strongly positive; this makes them more attractive to the negative charge in the cloud.
STREAMERS AND LEADERS
As objects on the ground become more and more positive they begin to send out what’s known aspositive streamers.These positive streamers reach out toward the cloud trying to make a connection. At the same time, astep leaderis moving down from the cloud. This is a narrow channel coming down from the base of the cloud, forming a zigzag pattern as it builds towards the ground. This step leader or channel is filling with electrons as it makes its way to the ground. Once the step leader gets close to the ground, the positive streamers try to connect up with it. Once one of them makes the connection, the channel is complete and all the electrons can now flow. This whole process, up to this point, will typically take about one second.
The electrons in the channel that are closest to the ground will begin to flow first, followed by electrons farther and farther up the channel. As these electrons flow they bump into particles of air, transferring some of their energy in the form of heat. This causes the air to heat up and glow. Since the electrons flow from the bottom up, it may appear that the lightning originated from the ground even though it originated in the cloud. If there is a large enough charge in the cloud we may see two or three “dumps” of electrons down the original channel. On the ground we would see this as a multiple flash of lightning.
Well, that’s all the room I have for this week. Next week we’ll continue our look at lightning by examining different types of lightning.
Sincetheelectronsflowfromthebottomup,it mayappearthatthelightningoriginatedfromthe groundeventhoughitoriginatedinthecloud.